Steve Ballmer just can’t walk away. It has long been obvious that linking with Yahoo is the Microsoft chief executive’s best chance of narrowing Google’s runaway lead in internet-based computing. That area is vital for immediate online revenue growth, both in paid search and display advertising. It is also important for Microsoft to protect itself against threats to its core Windows and Office franchises from Google in the future.

So, only days after attempting to remain disciplined on price and break off its pursuit of Yahoo, Microsoft is back. After all, its worst nightmare would be to find that its original approach had pushed Yahoo into linking with Google in online advertising. Such a move would have antitrust risks. But, if the two market leaders could pull it off, Microsoft would be left nowhere in an area that is pivotal for the future.

The software group is used to getting its way. It must have felt very uncomfortable walking away from the most attractive prospect in town, only to watch it cosying up to arch-rival Google. Carl Icahn’s proxy fight is an important catalyst. It puts the heat on Yahoo to maximise value for shareholders. Any deal it tries to strike with Google now would be heavily scrutinised to check that it did not simply rob shareholders of a takeover premium in the future. At the same time, Mr Icahn’s move gives Microsoft an excuse to reopen discussions with Yahoo so quickly.

The latest idea being mooted of an alliance between Microsoft and Yahoo sounds unattractive. Clear control, through an acquisition, makes far more sense if the combined entity is to take on Google. Mr Icahn has given both sides a final chance. Much now depends whether Mr Ballmer and Yahoo’s chief executive, Jerry Yang, can swallow their pride and make it happen.